The subject invention is directed toward the art of sensing devices and, more particularly, to a device for indicating when fluid flow has exceeded a predetermined minimum.
The invention is especially suited for use as a gas flow switch to produce an electrical signal when gas flow through the device has reached a set level and will be described with reference thereto; however, as will become apparent, the invention could be embodied in other devices which, for example, provide a mechanical or pneumatic output signal.
Gas flow indicators of the general type under consideration are known in the art and are shown and described, for example, in U.S. Pat. Nos. 2,963,563; 4,181,835; and 4,313,111. The indicators shown in each of these patents uses a cylindrical magnetic piston slidably mounted in a cylindrical gas flow passageway to actuate a magnetic field responsive switch. Generally, the piston is sized to be only slightly smaller in diameter than the passageway to provide a narrow annular flow space between the piston and the passageway. When gas flow through the device reaches a predetermined level, the pressure build-up on the upstream end of the piston is sufficient to move the piston downstream against a spring or gravity bias. Movement of the magnetic piston actuates an associated reed switch or the like to provide an output signal indicative of a predetermined flow level.
Certain problems exist with these prior devices. For example, there is often an inconsistency present in the devices. In particular, when using a magnet to activate a reed switch, small variations in magnet strength or slight differences in the orientation of the reeds can produce relatively significant differences in the point at which actuation takes place.
The noted problem is further compounded in certain prior devices because their designs are such that movement of the piston in an actuation direction reduces the length of the flow restriction. Thus, as the piston begins moving toward the actuating point, greater and greater flow is required to keep the piston moving. This tends to result in a "dampening" of piston movement and leads to further inconsistencies in the point at which the switch is actuated.
In addition to the above, the prior devices inherently act as flow limiting restrictions in the flow lines and thereby prevent flow levels significantly in excess of the point at which the minimum flow indication is produced. This is an undesirable characteristic in certain systems. For example, in some manufacturing and processing systems a certain minimum gas flow may be required for one phase of system operation but a much larger flow for a subsequent phase of operation or for system purging. To achieve the large flow volume differences with the prior valves it was often necessary to install by-pass piping systems, multiple valve manifolds and the like.